It is known that in today's competitive sawmill environment, it is desirable to quickly process straight or non-straight cants so as to recover the maximum volume of cut lumber possible from the cant. For non-straight cants, volume optimization means that, with reference to a fixed frame of reference, either the non-straight cant is moved relative to a gangsaw of circular saws, or the gangsaw is moved relative to the cant, or a combination of both, so that the saws in the gangsaw may cut an optimized non-straight path along the cant, so-called curve-sawing.
Increasingly, many parts of the world are growing plantation forest of Radiata Pine, Eucalyptus and other fast growing species. One of the problems with these trees is that the heartwood has poor strength characteristics and is unsuitable for structural grade timber. Consequently, it is used for non-structural purposes such as pallet stock where its value is roughly one third that of structural lumber. Structural lumber is made from the outside, or sapwood of the tree.
A canted log, or “cant”, by definition has first and second opposed cut planar faces. In the prior art, cants were fed linearly through a profiler or gang saw so as to produce at least a third planar face either approximately parallel to the center line of the cant, so called pith sawing, or split taper sawing, or approximately parallel to one side of the cant, so called full taper sawing; or at a slope somewhere between split and full taper sawing. For straight cants, using these methods for volume recovery of the lumber can be close to optimal. However, logs often have a curvature and usually a curved log will be cut to a shorter length to minimize the loss of recovery due to this curvature. Consequently, in the prior art, various curve sawing techniques have been used to overcome this problem so that longer length lumber with higher recovery may be achieved.
Curve sawing typically uses a mechanical centering system that guides a cant into a secondary break-down machine with chipping heads or saws. This centering action results in the cant following a path very closely parallel to the center line of the cant. Cants that are curve sawn by this technique generally produce longer, wider and stronger boards than is typically possible with a straight only sawing technique where the cant being sawn has significant curvature. Boards that are cut using curve sawing techniques straighten out once they are stacked and dried.
Curve sawing techniques have also been applied to cut parallel to a curved face of a cant; the above mentioned full taper sawing. See for example Kenyan, U.S. Pat. No. 4,373,563 and Lundstrom, Canadian Patent No. 2,022,857. Both the Kenyan and Lundstrom devices use mechanical means to center the cant during curve sawing and thus disparities on the surface of the cant such as scars, knots, branch stubs and the like tend to disturb the machining operation and produce a “wave” in the cant. Also, cants subjected to these curve sawing techniques tend to have straight sections on each end of the cant. This results from the need to center the cant on more than one location through the machine. That is, when starting the cut the cant is centered by two or more centering assemblies until the cant engages anvils behind the chipping heads. When the cant has progressed to the point that the centering assemblies in front of the machine are no longer in contact, the cant is pulled through the remainder of the cut in a straight line. It has also been found that full taper curve sawing techniques, because the cut follows a line approximately parallel to the convex or concave surface of the cant, can only produce lumber that mimics these surfaces, and the shape produced may be unacceptably bowed.
Thus in the prior art, so called arc-sawing was developed. See for example U.S. Pat. Nos. 5,148,847 and 5,320,153. Arc sawing was developed to saw irregular swept cants in a radial arc. The technique employs an electronic evaluation and control unit to determine the best semi-circular arc solution to machine the cant, based, in part, on the cant profile information. Arc sawing techniques solve the mechanical centering problems encountered with curve sawing but limit the recovery possible from a cant by constraining the cut solution to a radial form.
Applicant is also aware of U.S. Pat. No. 4,572,256, U.S. Pat. No. 4,690,188, U.S. Pat. No. 4,881,584, U.S. Pat. No. 5,320,153, U.S. Pat. No. 5,400,842 and U.S. Pat. No. 5,469,904; all of which relate to the curve sawing of two-sided cants. Eklund, U.S. Pat. No. 4,548,247, teaches laterally translating chipping heads ahead of the gangsaws. The U.S. Pat. Nos. 4,690,188 and 4,881,584 references teach a vertical arbor with an arching infeed having corresponding non-active tilting saws and, in U.S. Pat. No. 4,881,584, non-active preset chip heads mounted to the sawbox.
U.S. Pat. No. 4,599,929 to Dutina teaches actively translating and skewing of gangsaws for curve sawing, where a saw guide package is adjusted. The saw axle may also be adjusted in view of the average inclination over the sawing line of the entire longitudinal profile of the workpiece or of parts of the longitudinal profile.
U.S. Pat. No. 4,144,782 to Lindstrom teaches that when curve sawing a log, the log is positioned so as to feed the front end of the log into the saw with the center of the log exactly at the saw blade. In this manner the tangent of the curve line for the desired cut profile of the log extends, starting at the front end, parallel with the direction of the saw blade producing two blocks which are later dried to straighten and then re-sawn in a straight cutting gang.
U.S. Pat. No. 5,884,682 to Kennedy et. al, discloses that optimized lumber recovery is best obtained for most if not all cants if a unique cutting solution is determined for every cant. Thus for each cant a “best” curve is determined, which in some instances is merely a straight line parallel to the center line of the cant, and in other instances a complex curve that is only vaguely related to the physical surfaces of the cant.
U.S. Pat. No. 5,722,474 to Raybon, et al. teaches using scanned data to saw a cant, by moving the cant through the gang sawbox while pivoting and translating the gang sawbox. The gang sawbox contains a fixed sawguide package to curve saw the curvature in the log.
U.S. Pat. Nos. 5,761,979 and 5,870,936 to McGehee disclose using a saw guide or saw guides where sawguides and saws are actively translated along a fixed driven arbor. The sawguides and saws may be skewed a few degrees on either side of the perpendicular to the arbor axis, so that the saws either actively traverse a non-symmetrical board fed into the saws lineally for optimum board edging, or actively follow a curved path for sawing boards from a cant fed into the saws lineally, from optimized data of the scanned profile. This system permits curve sawing without requiring the movement of the entire saw box.